The present invention relates in general to spectroscopic analysis of very low concentration of a substance having a known characteristic absorption spectrum in the presence of other substances having absorption spectra which overlap that of the substance to be measured. More particularly, the present invention is directed toward a wavelength-modulated derivative spectrometer for spectroscopic measurement of the concentration of a gas or vapor in the presence of another gas or vapor which has an absorption spectrum with a periodic structure such that the maxima and minima of this periodic structure are relatively narrow by comparison with maxima or minima of the absorption spectrum of the substance to be measured, so that the spectrum of the substance having a periodic spectral structure tends to overlap the spectrum of the substance to be measured and thereby interfere with such measurements. Such a situation is typified by the measurement of trace amounts of ammonia (NH.sub.3) in stack gas from an oil-fired furnace, such stack gas containing a relatively high concentration of sulfur dioxide (SO.sub.2).
Derivative spectrometry based on wavelength modulation is an extremely sensitive method of spectroscopic analysis, which has been described in various patent applications and scientific papers, for example in Japanese Pat. No. 033095, 1976, and in the Journal of physics E : Scientific instruments, Vol. 14, 1981, pp. 105 to 112 (published in the United Kingdom). Such prior art types of wavelength-modulated derivative spectrometer have been proved effective in the measurement of a spectrum having a relatively fine structure in the presence of an interfering substance having a relatively broad spectral structure. However such prior art wavelength-modulated derivative spectrometers cannot measure small concentrations of a substance having relatively broad spectral characteristics in the presence of a substance a periodic spectrum structure having more finely detailed spectral characteristics (i.e. having more sharply curved spectral maxima and minima). For this reason, such wavelength-modulated derivative spectrometers of the prior art cannot be used, for example, to measure low concentrations of ammonia in the presence of large amounts of sulfur dioxide, since the spectral characteristics of the sulfur dioxide produce a large amount of interference. There is at present a requirement for an accurate and reliable means for continuous measurement of ammonia concentration in the stack gas from various types of plant, e.g. from nitrogen oxide purging plant used with LPG, coal, or oil-fired plant, which has not hithereto been met in a simple and effective manner. Satisfactory means for meeting the above requirements are disclosed by the present invention, as will be described hereinafter with reference to specific embodiments.